Have you often wondered why some highway route signs seem to send you in the direction you just came from? Or in a direction you sense is wrong? You’ll seldom get lost following the signs, even if they do point north and you want to go east; fortunately, a misplaced directional sign is a rarity. The whole navigational task becomes much simpler, however, if you understand the two systems used to number federal highways.

All interstate expressways that are east-west routes have a letter-number-direction designation. For example, Interstate 80, throughout its length, is an east-west traffic corridor and all signs will be marked correspondingly as “I-80 East” or “I-80 West,” even on those occasional sections that for a short distance head straight north or south. So don’t turn off the highway and risk getting lost, even if you thinking the direction is totally wrong in relation to where you want to go.

The numbering of east-west interstate highways also depends on their north-south location. The numerical order begins with Interstate 4, in Florida, and ends with I-96, in Michigan, the one farthest north. The north-south interstates are odd-numbered, with the numbers getting higher from west to east; Interstate 5 is in California, Interstate 95 in Virginia.

With this east-west and north-south numbering system acting as a grid, it’s easy to visualize an interstate expressway’s location without a map.

Three-digit Interstate numbers are found in and around large metropolitan areas. Remember that prefixing an even digit to the route number-as in Interstate 235, for example- means that the expressway continues through or around a city. An odd number prefix means the route is a “spur” that ends in the downtown area.

Federal highways other than the interstate expressways also follow an east-west and north-south numbering system, but the ascending numerical orders are reversed-that is, the lower numbered U.S. highways will be on the east coast, and in the north near the Canadian border.